USPS rates are inaccurate in real time

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I decided to use the carrier rates for shipping through Shopify for UPS and USPS. It looks like UPS rates are accurate, but the USPS rates are not correct at all. For instance, a 15 lb package going to AL from VT is not $10 and change. I've tried out the shopping cart with multiple addresses and the priority mail rates are under estimated by a lot. What am I missing?